Telephone system



Aug. 8, 1939. E. s. PETERSON I TELEPHONE sYsTEM 2 Sheeis-Sheet l Filed Dec. 10, -1937 0 m2 IrC-200 OJ ATTORN E Y5 Aug. 8, 1939. E. s. PETERSON TELEPHONE SYSTEM Filed Dec. 10, 1937 2 Sheets-Sheet 2 INVENTOR. EDWARD S. PETERSON BY wvwpyduf, WZL m/ ATTORN E YS Patented Aug. 8, 1939 UNITED STATES PATENT OFFICE TELEPHONE ASYSTEIVI Application December 10, 1937, Serial No. 179,125

14 Claims.

The present invention relates to telephone systems and more particularly to improvements in small systems adapted for use in providing communication facilities between the several compartments or cars of a train or other articulated vehicle.

Small systems of the character noted conventionally comprise a plurality of telephone stations and a line circuit extending between the stations, together with switching apparatus at each station for selectively establishing connections between the transmitters and receivers located at the respective stations. In the particular application suggested above, there is great utility in arranging the system so that communication may `be had between the locomotive cab and the other cars, and particularly between the cab station and a station located in the rear car. By providing such communication facilities it is possible for the engineer to be at all times advised of conditions whichprevail at the rear of the train, which, of course, is of great assistance during backward movement of the train when the engineer has no clear view in the direction of movement. Due to the high noise level which ordinarily prevails in a locomotive cab` it has been found to be desirable to use as the receiving means in the cab, a vacuum tube power amplifier unit arranged to deliver its output to a loud speaker, so that voice frequency currents incoming to the cab are reproduced at a suiiiciently high acoustic level as to predominate over the noise level inherently present in the surroundings. The provision of such a unit creates the problem of eliminating regenerative effects in the system which tend to produce "singing or howling in the loud speaker output.

It is an object of the present invention therefore, to provide an improved system of the character described which is eflicient in operation, is of simple and economical arrangement, and is so arranged that regenerative effects are substantially eliminated.

One embodiment of the invention is illustrated as comprising a plurality of telephone stations, each including transmitting means and receiving means, which stations are interconnected by a line circuit comprising a plurality of conductors. Each station also includes a signal device and switching means comprising a plurality of manually operable switches and at least one relay. In accordance with one feature of the invention the switching means, the line conductors and the signal devices are so connected and. arranged that independent circuits, each including a pair of the conductors of the line are provided for causing the energization of the signal device at any one of the telephone stations in response to the selective operation of the switches at any of the other stations. By this arrangement, any

particular station may be called from any of the other stations by signalling over the line. Also, in accordance with the invention, the switching means, the conductors and the transmitting and receiving means are so connected and arranged that separate and substantially independent channels, each comprising a pair of line conductors may be provided for transmitting voice frequency currents between any two of the stations in the respective two directions of transmission through the selective operation of the manually operable switches. In order to eliminate regenerative effects, the windings of the various relays arel so connected and arranged that no coupling between channels is provided thereby when a transmission channel is established between the locomotive cab station and any other station.

Further features of the invention pertain to the particular arrangement of the circuit elements whereby the desired communication facilities are obtained with a minimum amount of apparatus.

The novel features believed to be characteristic of the invention are set forth with particularity in the appended claims. The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the specification, taken in connection with the accompanying drawings, in which one embodiment is illustrated. In the drawings, Figs. 1, 2, 3, and 4 illustrate a telephone system having incorporated therein the features of the present invention.

Referring now to the drawings, the telephone system there shown comprises a plurality of telephone stations, |00, 200 and 300, located, respectively, in the locomotive cab, the dining car and the rear car of a train. These stations have extending therebetween a line circuit or line comprising a plurality of conductors |0I, |02 and |08. As an adjuct of the station 300 provided in the rear car, there is also provided a rear car extension station 400, which is located at the extreme rear of the rearmost car, so that an-operator thereat will have a full view of the tracks lying to the rear of the train.

The apparatus located at the station |00 comprises transmitting means in the form of a transmitter HI, receiving means in the form of a tandem connected power ampliiier and loud speaker |60, a source of direct current, not shown, having its terminals connected to the line terminals |35, a signal device comprising a lamp |02 for indicating incoming calls to the station |00, and switching means comprising two manually operable key switches |20 and |3| and a relay ||0 for initiating calls to the other stations and for conditioning the transmitting and receiving means for operation. The switch |20 is of the non-locking type, but the switch |3| is of the locking type such that it remains in its oil-normal position when once operated to this position. It will be appreciated that the key switch effectively comprises two switches since separate and independent functions are performed when the actuating member of the switch is operated to its respective two off-normal positions.

The apparatus located at the station 200 comprises a telephone hand set including a transmitter 246, a receiver 241, and a hook switch lndicated generally at 240. This hook switch is included in the switching apparatus for initiating calls -to other stations and for connecting the associated transmitter and receiver to the line circuit. 'I'he switching apparatus also includes a pair of manually operable non-locking key switches 220 and 230, which are arranged respectively to be used to initiate calls to the rear car station 300 and the locomotive cab station |00. Also forming a part of this apparatus is a relay 2|0, which, in combination with the key switch 220, functions properly to associate the transmitter 246 and the receiver 241 with the conductors of the line circuit for communication between the station 200 andthe rear car stations 300 and 400. Current for energizing this relay is supplied from a direct current source, not shown, having its terminals connected to the conductor terminals 250. For the purpose of signalling the attendant at the station 200 on calls incoming to this station, there is provided a signal device or buzzer 231.

The apparatus located at the rear car station 300 is quite similar to that provided at the station 200 and just described, in that it comprises a telephone hand set including a transmitter 356, a receiver 351 and a hook switch 350. The switching apparatus at this station comprises, in addition to the hook switch noted, a pair of manually operable key switches 330 and 340, both of which are of the non-locking type, and a pair of relays 3|0 and 320. The first of these relays corresponds to the relay 2|0 at the station 200, in that its function is that of properly associating the transmitter 356 and the receiver 351 with the conductors of the line for communication between the stations 200 and 300. The sole function of the relay 320, on the other hand, is to control the operating circuit of a buzzer 330 forming the signal device at the station 300. Current for energizing the relays 3|0 and 320 and the buzzer 338 is supplied by a direct current source, not shown, having its terminals connected to the conductor terminals 341.

Turning now to a consideration of the apparatus located at the extension or station 400, this 'apparatus comprises a hand set including a transmitter 425 and a receiver 425, together with switching apparatus referred to below. The hand set is preferably located in a small compartment and is normally disconnected from the conductors of the line at the springs 42| and 422 forming a part of two switches 4|9.and 420, respectively. The actuating members for these switches are indicated schematically at 423 and 424, respectively, and are moved to open circuit the associated spring when the door of the compartment housing the hand set is closed. Ordinarily there is no occasion to use this station aside from those instances where a trainman at the rear of the car desires to talk with the engineer located at the station |00. Hence, there is only provided at the rear car extension station 400 switching apparatus operative to energize the signalling device located at the locomotive cab station III. Specifically, this apparatus comprises a manually operable key switch 4|8 for controlling the ener'- glzation of a relay 4|0 from 'a direct current source, not shown, having terminals connected lto the conductor terminals 4|5. y

Each of the transmitters |4|, 246, 356 and 425 is of the self-powered type which, when driven by voice frequency sound waves, develops a sufficient voltage output to eliminate the necessity for using an auxiliary currenty source as a vehicle for voice transmission. Several well-known transmitters of this type are commercially available. In brief, this type of transmitter usually comprises a diaphragm which is connected by means of a sturdy but very light connecting rod to an armature disposed between the poles of a very powerful permanent magnet. When sound waves impinge upon the diaphragm it vibrates in accordance with these waves and transmits its vibration to the armature. The armature, in turn, vibrates and sets up variations in the magnetic lines of force, developed by the permanent magnet, both in number and direction, causing an alternating voltage to be induced in the coil windings forming a part of the magnetic field. The induced voltage thus generated varies in frequency and amplitude in accordance with the input sound waves.

Although any desired type of power ampliiler may be used as the unit |50, this unit is preferably arranged as disclosed in copending application, Serial No. 184,253, filed Jan. l0, 1938, Roswell H. Herrick. 'I'he remaining circuit elements, referred to above, may be of any desired commercial form. Electrically independent current sources, preferably storage batteries, are provided in the respective cars for energizing the various signal devices and control relays in the manner pointed out moreparticularly hereinafter.

Considering now the operation of the apparatus to establish a talking connection between the station 200 and the station |00 on a call initiated at the station 200. when the hand set comprising the transmitter 246 and the receiver 241 is removed from its cradle the hook switch 240 operates to close its contacts, thereby to complete a path for connecting the transmitter 246 between the line conductors |0| and |02. This path extends from the conductor |0| by way of the switch springs 222 and 223, the contact 2|2, the armature 2||, the direct current blocking condenser 249, the transmitter 246, the hook switch springs 243, 244 and 245, and the conductor 28|, to the opposite line conductor |02. When the hook switch 240 operates, a path is also complete'd for connecting the receiver 241 between the line conductors |02 and |03, this path extending from the conductor |02 by way of the conductor' 26|, the hook switch springs 245 and 244, the receiver 241, the blocking condenser 240. the armature 2|4, the contact 2|5, and the conductor 262 to the opposite line conductor |03. Following the removal of the hand set from its cradle, the calling party may challenge the line to determine whether or not it is idle. If the line is idle the calling party may signal the called station |00 by operating the key switch 230 to its off-normal position to impress the voltage of the current source provided at the station 200 across the line conductors |02 and |03. More specifically, this operation results in the completion of a circut for energizing the relay ||0 located at the station |00. This circuit extends from the Side of the current source located at the station 200 which is connected to the terminal 25|, by way of the switch springs 235 and 236, the conductor 262, the line conductor |03, the switch springs |26 and |29, the upper winding of the relay ||0, the switch springs |23 and |24, the line conductor |02, and the switch springs 23| and 232, to the opposite terminal of the source as connected to the terminal 252. The relay ||0, upon operating, completes a circuit for energizing the signal lamp |32, this circuit extending from the terminal |36, which is connected to one side of the current source provided at the` station |00, by way of the armature the signal lamp |32 and the current determining resistor |33 to the opposite side of the source as connected to the terminal |31. Each time the key switch 230 is actuated to its off-normal position, the relay ||0 is caused to operate, thereby to energize the lamp |32. 'I'he energization of this lamp indicates to the engineer at the station |00 that an incoming call awaits attention.

Although the desired connection is completed each time the relay ||0 operates in response to the operation of the key switch 230, it is necessary for the engineer at the station |00 to operate the key switch |3| to its off-normal position to complete an alternative circuit for energizing the relay ||0 if the talking circuit is to be held completed independently of the switch 230. The alternative circuit noted extends from one side of the current source as connected to the terminal |36 by way of the springs oi the switch |3|, the lower winding of the relay ||0 and back to the opposite side of the source at the terminal |31.

The switch |3| is of the locking type, such that when actuated to its oi-normal position it remains in this position until it is manually restored to its normal position and, hence, the relay ||0 is held energized when this switch is operated to its olf-normal position. With the relay ||0 operated, the transmitter |4| is connected by means comprising the blocking condenser |40 and the armature ||5, between the line conductors |02 and |03. With this relay operated, there is also completed a path for connecting the input terminals of the power ampliiier |50 between the line conductors |0| and |02, this path extending by way of the blocking condenser |39, the armature |2, and the contact I3. It is to be observed that with the apparatus in this condition and with the power amplifier |50 operating, voice frequency current transmission channels are established for conducting voice frequency'currents between the two stations |00 and 200. Specifically, the power ampliiier |50 and the transmitter 246 are connected between the line conductors |0| and |02, and the receiver 241 and the transmitter |4| are connected between the line conductors |02 and |03. Thus, two separate and substantially independent channels are provided for voice frequency currenttransmission in the respective two directions of transmission.

When the conversation between the train attendant at. the station |00 and the calling party at the station 200 is terminated, the connection may be released through the actuation of the switch |3| to its normal position, and the restoration of the hand set comprising the receiver 241 and the transmitter 246 to its cradle. This latter operation causes the contacts oi' the hook switch 240 to be opened to disconnect the transmitter 2 46 and the receiver 241 from the common conductor |02'of the line circuit. When the switch |3| is restored to its normal position, the operating circuit for the relay III is interrupted, caus- Mmi.

ing this relay to restore to disconnect, at its armatures ||5 and ||2, the transmitter |4| and the ampliiler |50 from between the conductors of the line circuit and to open, at its armature the operatingcircuit for the signal device |32.

The completion of a connection between the rear car. station 300 and the cab station |00, is substantially similar to that just described. In brief, when the calling party removes the hand set at the station 300 from its cradle, the hook switch 350 is voperated to close its contacts. As a resulto! this operation, the transmitter 356 is connected between the line conductors |0| and |02 over a path extending from the ilrst mentioned conductor by way of the switch springs 332 and 333, the contact 3|2 and its associated armature 3||, the direct current blocking condenser 358, the transmitter 356, the springs 353, 354 and 355 and the conductors 362 and 364, to the line conductor |02. Simultaneously, a path is completed for connecting the receiver 351 between the conductors |02 and |03, this path extending from the conductor |02 by way of the conductor 364, the conductor 362, the springs 355 and 354, the receiver 351, the blocking condenser 359, the armature 3I5, the contact 3|4, the conductor 363, the contact 4|4, and the armature 4|2 to the line conductor |03. When the calling party thereafter actuates the cab call key 340 to its off-normal position, the voltage of the source at the station 300 is impressed between the conductors |02 and |03 over a path having one branch extending from one side of the source at the terminal 34B by,way of the springs 345 and 346, the conductor 363, the contact 4|4, and the amature 4|2 to the conductor |03, and a second branch extending from the opposite side of the source at the terminal 349 by way of the switch springs 34| and 342, and the conductor 364 to the opposite line conductor |02. This voltage is impressed, by the two line conductors noted, across the upper winding of the relay ||0 over the path described previously, causing this relay to operate to complate the energizing circuit for the signal lamp |32 and to establish the desired talking' channels. When the call is answered at the station |00 through the actuation of the switch |3| to its olf-normal position, the relay ||0 is locked operated to hold the input terminals of the amplifier |50 connected between the line conductors |0| and |02, and the transmitter |4| connected between the line conductors |02 and |03. With the transmitter 356, the receiver 351, the transmitter 34| and the power amplier |50 connected as described, two separate and substantially independent channels are provided for transmitting voice frequency currents in the respective two directions of transmission. Here again, the channel for transmitting voice frequency currents from the transmitter 356 to the power amplifier |50 comprises the first conductor |0| and the common conductor |02, while the channel for transmitting currents from the transmitter |4| to the receiver 351 comprises the common conductor |02 and the second conductor To release the connection as established between the stations 350 and |00, it is only necessary for the calling party to restore his hand set to its cradle, thereby to open the contacts of the hook switch 350 to disconnect the receiver 351 and the transmitter 356 from the line conductors, and for the called party to restore the switch |3| to its normal position, whereby the holding circuit for the relay ||0 is opened. When the relay restores the circuit for energizing the lamp |32 is opened at the amature and the transmitter I 4| and amplifier |50 are disconnected from across the conductor of the line circuit at the armatures ||2 and 5.

As pointed out above, a call to the cab station |00 may also be initiated at the rear car extension telephone station 400. If the telephone at this station is to be used, -the calling party removes the hand set comprising the transmitter 425 and the receiver 426 from its retaining compartment and in so doing necessarily opens the door of this compartment to cause the contacts of the two door actuated' switches 4|9 and 420 to be closed. With the contacts 42| closed, an obvious path, including the direct current blocking condensers 421 and 423 is completed for connecting the transmitter 425 between the line conductors |0| and |02. Similarly, with the contacts 422 of the switch 420 closed, an obvious path, including the condenser 428 and the conductor 429, is completed for connecting the receiver 426 between the line conductors |02 and |03. To energize the signal lamp |32 at the cab station |00, thereby to indicate to the engineer that the station |00 is being called, the calling party may repeatedly actuate the nonlocking key switch 418 to close an obvious circuit for energizing the relay 4| 0. This relay, upon operating, connects, at its armatures 4|I and 4| 2 the line conductors |02 and |03 to opposite terminals 4|6 and 4|1, respectively, of the direct current source provided in the rear car. As a result, the upper winding of the relay 0 is energized over a circuit similar to that traced previously, to cause the energization of the signal lamp |32. Following observation of the energized condition of the signal device |32, the engineer, at the station |00 may, by actuating the key 3| to its off-normal position, cause the twovoce frequency current transmission channels to be locked completely in the manner described previously. Specifically, these channels are completedwhen the relay ||0 operates to connect the input terminals of the power ampliiier between the line conductors |0| and |02 and the transmitter |4I between the line conductors |02 and |03, and are held completed so long as the switch |3| is maintained operated. In this connection it is pointed out that the key switch 4|6 is released and allowed to restore when the call is answered. Hence, operation of the locking key switch |3| is necessary to maintain the relay ||0 operated. To release the connection as established in the manner set forth above, the calling party restores the hand set at the station 400 to its retaining compartment, and closes the door of this compartment, thereby to open the contacts 42| and 422, respectively, of the switches 4|! and 420. 'I'his results in the disconnection of the transmitter 425 and the receiver 426 from `the line conductors |02 and |03, respectively.

At the station |00, the engineer restores the key |3| to its normal position to open the holding circuit for the relay ||0, which relay, upon restoring, opens the operating circuit for the signal lamp |32 and disconnects the transmitter |4| and the power amplifier |50 from the line conductors |02 and |03, respectively.

A call from the locomotive cab station 00 to the rear car station 300 may be initiated by oper- 'ating the key switch |20 to its left oir-normal position, thereby to complete a circuit for energizing the relay 320. This circuit extends from the positive terminal |36 of the source provided at the station |00 by way of the switch springs |21 and |26, the line conductor |0|, the armature 3l3, the winding of the relay 320, the switch springs 344 and 343, the conductor 363, the contact 4|4, the armature 4|2, the conductor |03, the switch springs |29 and |30 and through the current determining resistor |34 to the negative terminal |31 of the current source. Before this operation is performed, the key switch |3| may be operated to its off-normal position to cause the operation of the relay ||0 in the manner described above, whereby the input terminals of the power amplifier |50 and the transmitter |4| are connected to the respective conductors |02 and |03 of the line. Through this operation, the calling party at the station |00 may ascertain whether or not the line is busy in connection with an ensuing conversation between the stations 200 and 300. Y

When the relay 320 operates, it completes, at its armature 32|, a circuit for energizing the buzzer 33 8, thisfcircuit extending from the negative terminal 349 of the current source provided at the station 300 by way of the armature 32|, the operating winding of the buzzer 338 and the current .determining resistor 3|9 to the positive terminal 348 of the current source. Through the operation of the buzzer 338, the rear car attendant is informed that a call awaits attention. This call may be answered at the station 300 or at the extension station 400 as desired. Assuming that the call is answered at the station 300, the answering party, by lifting the hand set from its cradle,

causes the operation of the hook switch 350 to complete the above-traced paths whereby the transmitter 356 is connected between the line conductors |0| and |02 and the receiver 351 is connected between the line conductors |02 and |03. Thus, the two voice frequency current transmission channels, as described above, are completed between the stations |00 and 300.

Assuming that the call to the rear car is answered at the extension station 400, it will be apparent that, when the hand set comprising the transmitter 425 and the receiver 426 is removed from its retaining compartment, the contacts 42| of the door actuated switch 4|9 are closed to conncct the transmitter 425 between the line conductors |0| and |02, and the contacts 422 of the door ractuated switch 420 are closed to connect the receiver 426 between the line conductors |02 and |03. As thus connected, two voice frequency current transmission channels are established which are substantially the same as those set up following the operation of the hook switch 350 to its off-normal position.

The key switch |20 is of the non-locking type such that it automatically restores to its normal position following its manual operation to the off-normal position. ,When the call is answered at the station 300 or the station 400 as the case may be, the calling party may release the switch, whereupon it assumes its normal position to open,

`at its springs |26 and |21, and its springs |29 and |30, two points in the above-traced operating circuit for the relay 320. Obviously, with this relay deenergized, the operating circuit for the buzzer 336 is interrupted. Hence, when the connection is finally established between the station |00 and the station 300 or the station 400, only the key switch |3| occupies its oit-normal position. When this switch is restored to normal by the calling party, the above-traced operating circuit for the relay ||0 is interrupted, causing this vtendant at the station 200.

relay to restore, to disconnect the power amplifier from across the line conductors and |02 and to disconnect the transmitter |4| from across the line conductors |02 and |03. It the call is answered at the station 300, the disconnect operation at this station is automatically performed when the hand set is restored to its cradle to restore the hook switch' 350 to its normal position, whereby the transmitter 356 is disconnected at the springs 354 and 355 from across the line conductors |0I and |02 and the receiver 351 is disconnected at the same set of springs from across the line conductors |02 and |03. If, on the other hand, the call is answered at the extension station 400, the disconnect operation is attained by restoring the hand set at this station to its retaining compartment and closing the door of this compartment to open the contacts 42| of the switch 4|9 and the contacts 422 of the switch 420. 'I'hrough the operation of the two door actuated switches 4|9 and 420, the transmitter`425 is disconnected from between the line conductors |0| and |02 and the receiver 426 is disconnected from across the line conductors |02 and |03.

In order to call the dining car station 200 from the cab station |00, the engineer may, after having operated the key switch |3| to its oir-normal position to cause the operation of the relay ||0 and the resulting connection oi' the transmitting and receiving means to the line conductors |02 and |03, operate the non-locking key switch to its right oir-normal position to complete a circuit for energizing the buzzer 231 at the station 206. This circuit extends 'from the positive terminal |36 of the source provided at the station |00 by way ot the switch springs |22 and |2|, the line conductor |0|,l the resistor 230, the armature 2|3, the operating winding oi' the buzzer 231, the switch springs 234 and 233, the line conductor |02 and the switch springs |24 and |25 to the negative terminal |31 of the source. Each time the key switch |20 is operated to its right off-normal position, the buzzer 231 operates to signal the at- When the call is answered, the key switch 20 is released so that the operating circuit for the buzzer 231 is left open. The connection is completed when the attendant at the station 200 removes the hand set at this station from its cradle to cause the operation f the hook switch 240 to its oil-normal position, thereby to connect the transmitter 246 between the line conductors |0| and |02 and the receiver 241 between the line conductors |02 and |03. To release the connection, it is only necessary that the key switch |3| be restored to normal, and that the hand set at the station 200 be replaced in its cradle. When the switch |3| is -moved to its normal position, the relay ||0 restores to disconnect the transmitter |4| from across the line conductors |02 and |03 and to disconnect the input terminals of the ampliiier |50 from across the line conductors |0| and |02.

When the attendant at the station 200 replaces f the hand set at this station upon its cradle, the

tion 200, the calling party may remove the handv set provided at this station from its associated cradle, thereby to cause the transmitter 246 to be connected across the line conductors |0| and |02 and the receiver 241 to be connected across the line conductors |02 and |03 in the manner set forth above. 'Assuming the line to be idle, the rear car may be signalled by operating the key switch 220 to its ofi-normal position, thereby to complete a circuit for energizing the relay 320, this circuit extending from the positive terminal of the source provided at the station 200 by way of the switch springs 22| and 222, the line conductor |0|, the armature 3I8, the winding of the relay 320, the switch springs 344 and 343, the conductor 363, the contact 4|4, the armature 4|2, the line conductor |03, the switch springs 221 and 226, and the resistor 239 to the negative terminal 252 of the source. When energized over the above-traced circuit, the relay 320 functions in the manner described previously to cause the operation of the buzzer 338, whereby the attendant in the rear car is informed that a call awaits attention. As pointed out above, the key switch 220 is of the non-locking variety and restores each time it is released, to interrupt the abovetraced operating circuit for the relay 320. When the call is answered, this switch is released and returns to its normal position to maintain the operating circuit for the relay 320 open. From the foregoing description, it will be apparent that the circuit connections are so arranged that normally the transmitters 246 and 356 are adapted to be connected across the same transmission channel, namely, between the conductors |0| and |02, and the receivers 241 and 351 are both both adapted to be connected across the other transmission channel, namely, between the line conductors |02 and |03. Thus, when the hook switch 240 is operated to its oli-normal position without any further operations being performed at the station 200, the transmitter 246 is connected between the line conductors |0| and |02 and the receiver 241 between the line conductors |02 and |03. Similarly, when the hook switch 350 is operated to iis oft-normal position, 'the transmitter 356 is connected between the line conductors |0| and |02 and the receiver 351 between the conductors |02 and |03. In order, therefore, to establish separate and substantially independent channels for voice frequency current transmission between the two stations 200 and 300, it is necessary to reverse the connections of the transmitter and receiver, at one of the two stations. In the present assumed case, this is automatically achieved immediately the key switch 220 is operated to its oir-normal position. This operation results in the completion of a circuit for energizing the relay 2I0, which circuit extends from the positive terminal 25| of the current source provided at the station 200, by way of the switch springs 225 and 224 and the windings of the relay 2|0 to the opposite terminal 252 of the source. The relay 2|0, upon operating, completes a holding circuit for itself, this circuit extending from the positive terminal 25| of the current source by way of the hook switch springs 242 and 24|, the armature 2|1 and through the winding of the relay 2 |0 to the opposite terminal 252 of the source. With the relay 2|0 operated, the circuit connections for the transmitter 246 and the receiver 241 are reversed from those which prevail when this relay is restored and the hook switch 240 is operated to its off-normal position. Thus, the transmitter 246 is now connected between the line conductors |02 and |03 over a lpath extending from the conductor |02 by way of the conductor 26|, the springs 245, 244 and 243, the transmitter 246, the blocking condenser 243, the amature 2|| and its associated contact 2|3 and the conductor 262 to the line conductor |02. The receiver 241, on the other hand, is connected between the line conductors |0| and |02 over a path extending from the rst-mentioned conductor by way of the contact 2|6 and its associated armature 2|4, the blocking condenser 243, the receiver 241, the springs 244 and 245 and the conductor 26| to the line conductor 02. With the transmitter 246 and the receiver 241 connected in this manner, and when the call to the rear car is answered at one oi.' the two stations 300 and 400, the desired separate and independent transmission channels are completed. Speciiically, when the hand set comprising the transmitter 336 and the receiver 351 is removed from its cradle to cause the operation of the hook switch 350 to its oit-normal position, the transmitter 356 is connected between the line conductors |0| and |02 over a path, previously traced, and, since the receiver at the station 200 is now connected between these same line conductors, a transmission channel extending between the station 300 and the station 200 is completed. Similarly, the receiver 351 is connected between the line conductors |02 and |03 and is now in circuit with the transmitter 246 at the station 200. It will be apparent that the attendant in the rear car may answer the call at the station 400 in preference to the station 300 by simply removing the hand set comprising the transmitter 425 and the receiver 426 from its retaining compartment. Since the transmitter and receiver of this hand set are, upon operation of the two switches 4`|3 and 420, connected between the same line conductors as the transmitter 3,56 and the receiver 351 at the station 300 following operation of the hook switch 350, it will be apparent that the desired two separate transmission channels are completed irrespective of which of the two stations 300 and 400 is the answering station.

When the calling party hangs up at the end of the conversation, the hook switch 240 is returned to itsnormal position to interrupt, at the springs 24| and 242, the holding circuit for the relay 2|0, causing this relay to restore to reprepare, at its armatures 2|| and 2|4, the originally traced paths by way of which the transmitter is connected between the conductors |0| and |02 and the receiver 241 is connected between the conductors |02 and |03. At the armature 2|1, a further point is opened in the holding circuit for the relay 2|0. At the springs 244 and 245, the transmitter 246 and the receiver 241 are disconnected from the line conductor |02. In view ofthe preceding description, it will be understood that, if the call is answered at the station 300 and the hand set at this station is returned to its cradle, the transmitter 356 and the receiver 351 are disconnected from the line conductor |02 through the operation of the hook switch 350 to open its contacts 354 and 355. Alternatively, if the call is answered at the station 400, the disconnect operation is attained when the hand set at this station is returned to its retaining compartment and the door to this compartment is closed to open the contacts 42| and 422 of the switches 4|3 and 420, respectively.

To call the dining car station 200 from the rear car station 300, the hand set at the lastmentioned station is rst removed from its cradle. This causes the hook switch 350 to connect the transmitter 356 between the line conductors |0| and |02 and the receiver 351 between -the line conductors |02 and |03. Thereafter, the calling party momentarily and repeatedly operates the key switch 330 to its ofi-normal position to signal the attendant at the station 200. Each time this non-locking key switch is operated to its otr-normal position, a circuit is completed for energizing the buzzer 231, this circuit extending from the positive terminal 343 of the current source provided at the station 300, by way of the springs 33| and 332, the line conductor |3I, the resistor 233,` the armature 2|3, the operating winding of the buzzer 231, the springs 234 and 233, the conductor 26|, the line conductor |02, the conductor 364 and the springs 331 and 336 to the negative terminal 349 of the source. At the conclusion of the signalling operation, the switch 333 is permitted to assume its normal position so that the above-traced circuit for energizing the buzzer 231 is held open. On this type of call, the reversal of the circuit connections whereby the two transmission channels are realized is at-4 tained through the operation of the relay 3|3. Thus, upon the rst actuation of the switch 333 to its oil-normal position, a circuit is completed for energizing this relay, this circuit extending from the positive terminal 343 of the current source provided at the station 300 by way of the springs 335 and 334 and through the winding of the relay 3|0 to the negative terminal 343 of source. Immediately this relay operates, it completes a holding circuit for itself which is independent of the switch 330. This holding circuit extends from the positive terminal 343 of the current source by way of the springs 352 and 36|, the armature 3|1 and through the winding of the relay 3|0 to the negative terminal 343 of the source. Thus, the relay 3|0 is locked operated until such time as the hand set at the station 300 is restored to its cradle. With the relay 3|0 in its operated position, the transmitter 353 is connected between the line conductors |32 and |03 and the receiver 351 is connected between the line conductors |0| and |02 to provide the required two channels for communication between the stations 200 and 300. Specifically, the transmitter 356 is now connected between the line conductors |02 and |03 over a path' extending from the first-mentioned conductor by way of the conductors 364 and 362, the hook switch springs 355, 354 and 353, the transmitter 356, the direct current blocking condenser 353, the arma- .ture 3|| and its associated contact 3|3, the conductor 363, the contact 4|4 and the armature 4|2 to the opposite line conductor |03. The path for connecting the receiver 351 between the line conductors |0| and |02 extends from the conductor |02 by way of the conductors 364 and 362, the switch springs 355 and 354, the receiver 351, the direct current blocking condenser 353, the armature 3|5 and its associated contact 3|3 to the line conductor |0|. When the hand set at the station 200 is removed from its cradle, the A transmitter 246 is connected between the line conductors |0| and 02 and the receiver 241 is connected between the line conductors |02 and |03 in the exact manner set forth above. Thus, the two transmission channels are completed and the parties may'converse over the established connection.

To release the connection as set up in the manner described in the preceding paragraph, it is only necessary for the calling and called parties to restore the two hand sets at the respective two stations to their respective associated cradles. When the hand set at the station 200 is restored to its cradle, the contacts of the hook switch 243 are opened to disconnect the transmitter 245 and the receiver 241 from the conductors of the line circuit. When the hand set at the station 300 is restored to its cradle, the holding circuit for the relay 3|0 is interrupted at the springs 35| and 352 of the hook switch 350, causing this relay to restore to open, at its armature 3|1, a further point in its holding circuit. They relay 3|0, upon restoring, reprepares, at its armatures 3|! and 3|5 and their respective associated contacts 3|2 and 3|li, the originally traced paths for connecting the transmitter 356 between the line conductors |0| and |02 and the receiver 351 between the line conductors |02 and |03. At the springs 354 and 355 of the hook switch 350, the transmitter 356 and the receiver 351 are disconnected from the conductor 02 of the line circuit and thus the line is cleared for further use.

From the foregoing description, it will be apparent that the switching means at the various stations, the line conductors and the signal devices are so connected and arranged that independent circuits each including a pair of the line conductors are provided for causing the energization of the signal device at any one station in response to the selective operation of the switches at the other stations. To summarize, va circuit comprising the line conductors |02 and |03 is utilized in signalling the station |00, a circuit including the line'conductors |0| and |02 is utilized in signalling the station 200, and a circuit comprising the line conductors |0| and |03 is used in signalling the station 300.

As pointed out in the introductory portion of the specification, one feature of the invention pertains to the substantial elimination of any input voltage to the amplier |50 occasioned by regeneration, or oscillation, of the system. This is partially achieved by the arrangement whereby separate and substantially independent voice Irequency current transmission channels are provided in a connection extending between any two of the stations. Another important factor in minimizing spurious input voltages to the amplifier |50 is the substantial elimination of any electrical or acoustical coupling between the two channels created when a talking connection to the station |00 is established. In this connection, it will be appreciated that the problem only arises when the station |00 is involved in a connection since the relay .I I0 is held deenergized to maintain the amplier |50 disconnected from the conductors of the line circuit when a talking connection is established between the two stations 200 and 300. When, however, a connection is established to the station |00, the input terminals of the amplier |50 are caused to be connected across the channel comprising the conductors |02 and |03. It is manifest that, if any coupling between this channel and the other channel, comprising the conductors |02and |03, were present, the voltage input to the amplifier |50 occasioned by such coupling would be stepped up by the amplier |50 and reproduced at a substantial volume by the loud speaker |60. The elimination of acoustical coupling between the channels is attained by maintaining suilicient spacing between the speaker |60 and the transmitter |4| to render the input to the latter negligible With a normal volume output from the speaker, and by holding the output volume at any of the receivers 241, 351 and 425 sufficiently low that a negligible input to the respective associated transmitters 24'5, 356 and 425 results even though the hand sets including the same are held away from the heads of the users to permit direct sound transmission between the elements of the respective hand sets. This elimination of acoustical coupling prevents the system from oscillating.

The elimination of electrical coupling between the two channels is achieved by the particular arrangement of the circuit elements involved in the' station signalling operations. Thus, when a connection is established between the station |00 and any one of the stations 200, 300 and 400, the upper winding of the relay I0 is connected across the line conductors |02 and |03 and is not coupled in any way to any circuit element included in or connected across the channel comprising the conductors |0| and |02. Similarly, the operating winding of the buzzer 231 is connected across the line conductors i0! and |02 and is in no way coupled to any circuit element included in or connected across the channel comprising the conductors |02 and |03. The only impedance element which is connected to provide coupling between the two channels is the winding of the relay 320 which is connected between the line conductors |0| and |03. This winding is, however, constructed to have an extremely high inductive impedance at frequencies within the voice frequency range and, hence, provides practically no undesired coupling between the two channels. Finally, the common conductor |02 of the line circuit is necessarily of negligible resistance, so that no substantial resistive coupling between the two channels is occasioned by including this conductor in each of the two channels. Thus, the particular circuit arrangement employed renders it possible to include the power ampliier |50 and its associated loud speaker in the system without introducing undesired side tone and singing effects during operation of the system.

While there has been described what is at present considered to be the preferred embodiment of the invention, it will be understood that various modications may be made therein, and it is contemplated to cover in the appended claims all such modiications as fall within the true spirit and scope of the invention.

I claim:

1. In a telephone system, a pair of telephone stations each including transmitting means and receiving means, a line circuit extending between said stations, said line circuit comprising a first conductor, a second conductor and a common conductor, a manually operable switch at one of said stations, means including said switch for connecting the transmitting means at said one station between said ilrst conductor and said common conductor and for connecting the associated receiving means between said second conductor and said common conductors, a second manually operable switch at said one station, a manually operable switch at the other of said stations, a relay at said other station, circuit connections controlled by said relay for connecting the transmitting means at said other station between said second conductor and said common conductor and for connecting the associated receiving means between said rst conductor and said common conductor, whereby separate and substantially independent channels may be provided for transmitting voice frequency currents between said stations in the respective two directions of transmission, a circuit including the switch at said other station for energizing said relay, a second circuit for energizing said relay, said second circuit including said second switch and said second and common conductors, a signal device at each of said I ceiving means, a line circuit extending between said stations, said line circuit comprising a first conductor, a second conductor and a common conductor, a manually operable switch at one of said stations, means including said switch for con-1, necting the transmitting means at said one sta` tion between said first conductor and said common conductor and for connecting the associated receiving means between said second conductor and said common conductor, a second manually operable switch at said one station, a manually operable switch at the other of said stations, a relay at said other station circuit connections controlled by said, relay for connecting the transmitting means at said other station between said second conductor and said common conductor and for connecting the associated receiving means between said iirst conductor and said common conductor, whereby separate and substantially independent channels may be provided for transmitting voice frequency currents between said stations in the respective two directions of transmission,a circuit including the switch at said other station for energizing said relay, a second circuit for energizing said relay, said second circuit including said second switch and said second and common conductors, a signal device at each of said stations, a second manually operable switch at said other station, means comprising a circuit including said first conductor and contacts controlled by said last-named switch for energizing the signal device at said one station, a circuit for energizing the signal device at said otherfstatlon, and means comprising contacts controlled by said relay for completing said last-named circuit.

3. In a telephone system, a pair of telephone stations, a transmitter and a receiver at one of said stations, a transmitter at the other of said stations, an amplifier and a reproducing device at the other of said stations, said amplier including input terminals and being connected to deliver its output to said reproducing device, a line circuit extending between said stations, said line circuit comprising a first conductor, a second conductor and a common conductor, a manually operable switch at said one station, means including said switch for connecting the transmitter at said one station between said first conductor and said common conductor and for connecting the associated receiver between said second conductor and said common conductor, a second manually operable switch at said one station, a manually operable switch at said other station, a relay at said other station, circuit connections controlled by said relay for connecting the transmitter at said other station between said second conductor and said common conductor and fpr connecting the input terminals of said ampliiier to said first conductor and said common conductor, respectively, whereby separate and substantially independent channels may be provided for transmitting voice frequency currents between said stations in the respective two directions of transmission, a circuit including the switch at said other station for energizing said relay, a second circuit for energizing said relay, said second circuit including said second switch and said second and common conductors, a signal device at each of said stations, a second manually operable switch at said other station, means comprising a circuit including said iirst conductor and contacts controlled by said last-named switch for energizing the signal device at saidone station, a circuit for energizing the signal device at said other station, and means comprising contacts controlled by said relay for completing said last-named circuit.

4. In a telephone system, a pair of telephone stations each including a transmitter and a receiver, aline circuit extending between said stations, said line circuit comprising a first conductor, a second conductor and a common conductor, switching means at each of said stations for connecting the respective associated transmitters between said first conductor and said common conductor and for connecting the respective associated receivers between said second conductor and said common conductor, a signal 'device at each of said stations, a relay at each of said stations, the relay at each station being operative to disconnect the associated transmitter from said first conductor and to connect it between said second and common conductors and also being operative to disconnect the associated receiver from said second conductor and to connect it between said first and common conductors, whereby separate and substantially independent channels may be established for transmitting voice frequency currents between Ysaid stations in the respective two directions of transmission-a manually operable switch at one of said stations, and means including said switch for energizing the relay at said one station and for energizing the signal device at the other of said stations.

5. In a telephone system, apair of telephone stations each including a transmitter and a receiver, a line circuit extending between said stations, said line circuit including a iirst conductor, a second conductor and a common conductor, switching means at each ofsaid stations for connecting the respective associated transmitters between said first and common conductors and for connecting the respective associated receivers between said second and common conductors, a relay at each of said stations, the relay at each station being operative to disconnect the associated transmitter from said first conductor and effectively to connect it between said second and common conductors and also being operative to disconnect the associated receiver from said second conductor and eiiectively to connect it between said iirst and common conductors, whereby separate and substantially independent channels may be established for transmitting voice frequency currents between said stations in the respective two directions of transmission, a signal device normally connected between said iirst and common conductors and located at one of said stations, a manually operable switch at the other of said stations, a source of potential at said other station, a circuit including said source for energizing the relay at said other station, and means comprising said switch for completing said last-named circuit and for connecting said source between said ilrst and common conductors, thereby to energize said signal device.

6. In a telephone system, a pair of telephone stations each including a transmitter and a receiver, a line circuit extending between said stations, said line circuit including a first conductor, a second conductor and a common conductor, switching means at each of said stations for connecting the respective associated transmitters between said first and common conductors and for connecting the respective associated receivers between said second and common conductors, a relay at each of said stations, the relay at each station being operative to disconnect the associated transmitter from said first conductor and effectively to connect it between said second and common conductors and also being operative to disconnect the associated receiver from said second conductor and effectively to connect it between said first and common conductors, whereby separate and substantially independent channels may be established for transmitting voice frequency currents between said stations in the respective two directions of trans` mission, a signal device normally connected between said first and common conductors at one of said stations, a manually operable switch at the other of said stations, a source of potential at said other station, a circuit including said source for energizing the relay at said other station, means comprising said switch for completing said last-named circuit and for connectingy said source between said rst and common conductors, thereby to energize said signal device, a holding circuit for maintaining the relay at said other station energized, means comprising contacts controlled by said last-named relay for completing said holding circuit, and means at said other station for interrupting said holding circuit.

7. In a telephone system, a pair of telephone stations each including a transmitter and a receiver, a line circuit extending between said stations, said line circuit including a first conductor, a second conductor and a common conductor, switching means at each of said stations for connecting the respective associated transmitters between said first and common conductors and for connecting the respective associated receivers between said second and common conductors, a relay at each of said stations, the relay at each station being operative to disconnect the associated transmitter from said first conductor and effectively to connect it between said second and common conductors and also being operative to disconnect theassociated receiver from said second conductor and effectively to connect it between said first and common conductors, whereby separate and substantially independent channels may be provided for transmitting voice frequency currents between said stations in the respective two directions of transmission, a signal device normally connected between said iirst and common conductors at one ofsaid stations, a manually operable switch at the other of said stations, a source of potential at said other station, a circuit including said source for energizing the relay at said other station, means comprising said switch for complet-v ing said last-named circuit and for connecting said source between said rst and common conductors, thereby to energize said signal device, a signal device at said other station, a normally incomplete circuit including said current source for energizing said last-named signal device, a second relay at said other station and operative to complete said last-named circuit, said second yrelay including a winding normally connected between said first and second conductors, a source of current at said one station, a circuit including said source for energizing the relay at said one station. a manually operable switch at said one station, and means including said iastnamed switch for completing said last-named circuit and for connecting said last-named source between said rst and third conductors, thereby to energize said second relay.

8. In a telephone system, a pair of telephone stations each including a transmitter and a receiver, a line circuit extending between said stations, said line circuit including a first conductor,

A a second conductor and a common conductor, switching means at each of said stations for connecting the respective associated transmitters between said first and common conductors and the respective associated receivers between said second and common conductors, and means at each of said stations for disconnecting the respective associated transmitters from said rst conductor and for connecting the same between said second and common conductors, said lastnamed means also being operative to disconnect the respective associated receivers from said second conductor and to connect the same between said first and common conductors, whereby separate and substantially independent channels may be provided for transmitting voice frequency currents betweenfsaid stations in the respective two directions of transmission.

9. In a telephone system, a pair of telephone stations each including a transmitter and a receiver, a line circuit extending between said staltions, said line circuit including a first conductor, a second conductor and a common conductor, switching means at each of said stations for connecting the respective associated transmitters between said rst and common conductors and for connecting the respective associated receivers between said second and common conductors, a relay at one of said stations operative to disconnect the associated transmitter from said first conductor and to connect it between said second and common conductors, said relay also being operative to disconnect the associated receiver from said second conductor and to connect it between said iirst and common conductors, a circuit for energizing said relay, a manually operable switch at said one station operative to complete said circuit, a relay at the other of said stations operative to disconnect the associated transmitter from said rst conductor and to connect it between said second and common conductors, said last-named relay also being operative to disconnect the associated receiver from said second conductor and to connect it between said iirst and common conductors, a circuit for energizing said last-named relay, and a manually operable switch at said other station operative to complete said last-named circuit.

l0. In a telephone system, three telephone stations each including transmitting and receiving means, a line circuit extending between said stations, said line circuit including a first conductor, a second conductor and a common conductor, switching means at the first of said stations for connecting the transmitting means thereat between said second and common conductors and for connecting the receiving means thereat between said first and common conductors, switching means located respectively at the second and third of said stations for connecting the respective associated transmitting means between saidI rst and common conductors and for connecting the respective associated receiving means between said second and common conductors, whereby separate and substantially independent channels may be provided for transmitting voice frequency currents between either of said second and third stations and said iirst station in the respective two directions of transmission, and means at each of said second and third stations for disconnecting the associated transmitting means from said first conductor and for connecting the same between said second and common conductors and for disconnecting the associated receiving means from said second conductor and for connecting the same between said rst and common conductors, whereby separate and substantialy independent channels may be provided for transmitting voice frequency currents between said second and third stations in the respective two directions of transmission.

11. In a telephone system, three telephone stations each including a transmitter and a receiver, a line circuit extending between said stations, said line circuit including a first conductor, a second conductor and a common conductor, switching means at the first of said stations for connecting the transmitter thereat between said second and common conductors and for connecting the receiver thereat between said first and common conductors, switching means located respectively at the second and third of said stations for connecting the respective associated transmitters between said first and common conductors and for connecting the respective associated receivers between said second and common conductors, a signal device at each of said stations, circuit connections including said conductors and so arranged that the signal device at any one of said stations may be energized independently of any of the other similar devices from any of the other stations, means at said second station for disconnecting the associated transmitter from said iirst conductor and for connecting it between said second and common conductors, said lastnamed means also being operative to disconnect the associated receiver from said second conductor and to connect it between said first and common conductors, and means at said third station for disconnecting the associated transmitter from said iirst conductor and for connecting it between said second and common conductors, said lastnamed means also being operative to disconnect the associated receiver from said second conductor and to connect it between said rlrst and common conductors.

12. In a telephone system, three telephone stations each including a transmitter and a receiver, a line common to and extending between all said stations, said line including a rst conductor, a second conductor and a common conductor, switching means at the rst of said stations for connecting the transmitter thereat between said second and common conductors and for connecting the receiver thereat between said first and common conductors, switching means located respectively at the second and third of said stations for connecting the respective associated transmitters between said first and common conductors and for connecting the respective associated receivers between said second and common conductors, whereby separate and substantially independent channels may be provided for transmitting voice frequency currents between either of said second and third stations and said first station in the respective two directions of transmission, .a signal device at each of said stations,

means including a circuit comprising said second `circuit comprising said rst and common conand common conductors for energizing the signal device at said nrst station, means including a circuit comprising said first and common conductors for energizing the signal device at said second station, and means including a circuit compricing said nrst and second conductors for energizing the signal device at said third station.

13. In a telephone system, three telephone stations each including a transmitter and a receiver,

a line common to and extending between all said stations, said line including a first conductor, a second conductor and a common conductor. switching means at the iirst of said stations for connecting the transmitter thereat between said second and common conductors and for connecting the receiver thereat between said rst and common conductors, switching means located respectively at the second and third of said stations for connecting the respective associated transmitters between said first and common conductors and for connecting the respective associated receivers between said second and com mon conductors, whereby separate and substantially independent channels may be provided for transmitting voice frequency currents between either of said second and third stations and said first station in the respective two directions of transmission, a signal device at each .of said stations, means including a circuit comprising said second and common conductors for energizing the signal device at said iirst station, means including a circuit comprising said iirst and common conductors for energizing the signal device at said second station, means including a circuit comprising said iirst and second conductors for energizing the signal device at said third station, and means at each of said second and third stations for disconnecting the associated transmitters from said first conductor and for connecting the same between said second and common conductors and for disconnecting the associated receivers from said second conductor and for connecting the same between said iirst and common' conductors, whereby separate and substantially independent channels may be provided for transmitting voice frequency currents between said second and third stations in the respective twodirections of transmission.

14. In a telephone system, three stations each including transmitting and receiving means, a,50 line common to and extending between said stations, said line comprising a first conductor, a second conductor and a common conductor, switching means respectively provided at said stations for connecting the respective associated. transmitting and receiving means to said line to provide separate and substantially independent channels for transmitting voice frequency currents between any ltwo of said stations in the respective two directions of transmission, a sig. nal device at each of said stations, means including a circuit comprising said second and common conductors for energizing the signal device at a iirst of said stations, means including a ductors for energizing the signal device at a second of said stations, and means including a circuit comprising said first and second convductors for energizing the signal device at the third of said stations. y ,70

EDWARD S. PETERSON. 

